Can-Am was so badass. Minimal rules, maximum speed and creativity. Minimum weight? Nah, we don’t need that. Maximum engine size? Nah, don’t need that either, and use turbos too if you like. Just have two seats, enclosed bodywork, and ‘reasonable’ safety to the design.
Porsche sort of got into Can-Am by accident. They had entered a 908 because of convenience (there happened to be a shared weekend with the 6-Hour Watkins Glen race) and did well, but were too far down on power to compete for a win. Further experiments with a spyder version of the 917K (near double the power of 908) were also too far down on power. The car looked like a good base, though, and the series a good challenge―and coupled with the additional North American exposure, Porsche decided to take a leap knowing there was one main problem: Power.
Solution? Turbocharging the 917’s engine out the wazoo.
The engine starts as the same 4.5L and 5.0L variants from the endurance models, but with compression ratio lowered significantly to 6.4:1, and two giant turbos from a diesel truck strapped on to produce in the neighborhood of 1.3-1.5bar boost.
At the 1.3bar race boost level, this makes for a 5.0L flat-12 outputting over 1,000hp@7800rpm and over 1000Nm torque in a range from 5,000-7,000rpm. Tighten some screws in the wastegate (this was early turbocharging, everything was mechanical) for some extra boost, and you get north of 1,250hp.
The cooling system struggles to keep up with this for anything more than short bursts, so it was common for drivers to back off on the boost during races to save the engine.
Gearbox was an evolution of the 917 unit, strengthened to handle all the extra torque, and reduced to a 4-speed. The differential was also a casualty of the design change. Tests using a spool on the 917K were positive for high-speed stability, and it removes another part which can break under the massive stress, so all the Can-Am 917s used a locked differential―something lead driver Mark Donohue really loved.
Aero development in the wind tunnel continued, and with all that extra power, drag could be sacrificed for the sake of downforce production. Late versions of the 917/10 tested at around 1,100lbf@150mph for downforce with a 75 percent rear balance and roughly 1.7:1 lift:drag efficiency.
The 917/10 would make its own weight in downforce at around 180mph. Impressive numbers for the time! All that downforce led to some suspension design changes to reduce camber change with the travel which was inevitable under load at high-speeds, plus adding back some anti-dive and anti-squat to the geometry to handle the braking and acceleration loads.
Minor differences from the 917K, or even 908/03, though. Old testing/race notes give some suspension stiffness targets they used; it ran very stiff at the rear to hold up almost a ton of downforce on the rear axle.
The chassis went through various lightening and strengthening phases, some even using a magnesium chassis frame that brought total weight below 800kg (1,200hp to move only 800kg!). That chassis was wiped out by Donohue in testing. The most successful/reliable cars from Penske weighed in at 820kgs and 837kg with 65percent on the rear axle.
To last through 200-mile races without refueling, the cars could hold 325L of fuel in tanks either side of the cockpit. With driver weight and full fuel load, weight distribution moves forward to 59.5 percent rear.
Tyres were fat…really fat. Rears are a full 17″ wide tread. Think of it this way: the car is about 217cm wide and the rear tyres combine for 110cm of that!
The driving dynamics are very much dominated by the rear-end and getting that power down. All that rubber plus a spool on the axle makes it understeer at low speed unless you run a very aggressive suspension setup with soft front and stiff rear.
Downforce increases with speed, and it’s balanced enough to the rear that it also adds some understeer for the sake of planting the rear tyres when the boost is on. With low rear wing settings and qualifying boost, it will spin the tyres up through 150mph.
Despite the tendency to understeeer, though, it is easy to rotate because there is always so much power on tap to help rotate the car. Be ready to use the brakes a lot. It accelerates so fast that you are approaching most turns 40-50mph faster than any other car of the era. It has good enough cornering grip, but the key to performance is getting into and out of the corners like the rocket it is. Harness that power and it will lap faster than an F1 car of the time.
Also, it does little wheelies on a hard launch. Raise the revs to build boost, dump the clutch, and it actually does a wheelie. This one is huge fun!
Jussi Karjalainen, Handling QA Lead
This thing is just a monster. You can’t steer it at all when off-throttle (unless you’re going really slow or abuse the transmission), so you must get the car to slow down and use a bit of throttle to help it rotate. You’re not really looking to use the throttle to turn it, but just to help it unlock. And guess what? The brakes aren’t much to shout about. And then there’s the 1,000+hp to deal with.
Initial tries at the Nordschleife put me at 6:32 and, at Classic Le Mans, at 2:55. That first value is as fast or faster than modern GT3s can handle at the Nordschleife. The second―the Le Mans time―is 20 seconds quicker than the 917 managed in 1971. Which was already 10 seconds quicker than the Ford Mk IV managed in 1967.